It is known that cross-linking of polymers substantially alters the physical properties of the polymers. Cross-linking can change a thermoplastic polymer to a thermoset polymer, can alter its solubility, density and other physical characteristics. Normally, cross-linking of a polymer is an irreversible process, so that the shape, configuration and density of a cross-linked polymer remain substantially permanent once the cross-linking process is complete.
A variety of different methods of polymer cross-linking are known. One method is reaction with chemical cross-linking reagents. This is particularly applicable where the starting polymer is unsaturated (polybutadiene, polyisoprene, styrene-butadiene copolymers, EPDM etc.), so that the groups of unsaturation take part in the cross-linking. Other methods involve creation of reactive sites such as free radicals on the polymer chains, e.g. by hydrogen abstraction using a free radical-generating initiator, by irradiation with γ-rays, X-rays, etc. Cross-linking can take place with the polymer in solution in a suitable solvent, in suspension or in bulk. Cross-linking is normally a random process, which may involve links between different polymer chains and links between points on the same polymer chain, and permits only limited control over its course and extent. In solution and suspension, non-cross-linked polymers tend to adopt an extended, coiled conformation, which is altered in a generally uncontrollable manner during cross-linking. There is a need for stable, solid particulate polymers of predetermined shape, size and density, for use for example in ink-jet printers, photocopiers and other imaging applications, where the achievement of fine definition and resolution of images depends upon the particle size and uniformity of the particles comprising the imaging medium, and on the viscosity of the imaging medium.